How long should your HIIT workouts actually be?

High-intensity interval training has been all the heart-racing rage ever since it, er, hit the fitness scene many moons ago.

But the sweat set might not realize that the most effective part of the workout is also the very thing the majority of people get wrong. “They think high-intensity, high volume—instead of high-intensity, low volume,” explains Rachel Vaziralli, the creative manager of group fitness at Equinox and exercise-science expert. “You can’t perform at a true high intensity if [the workout] is long.”

Too often HIIT is used as a synonym for “hardcore” to describe workouts that are 60 minutes or longer. Especially in boutique fitness classes, where intensity has become a near obsession, the term gets thrown around like a five-pound medicine ball. So what is the optimal amount of time to HIIT before you quit?

“If you can go longer than 30 minutes, you weren’t actually working hard enough.” —Rachel Vaziralli, creative manager of group fitness at Equinox

“If you can go longer than 30 minutes, you weren’t actually working hard enough,” Vaziralli says. The whole point, she explains, “is to push to unsustainable intensities.” (Having taken her new Firestarter class at the gym chain, I can attest that half an hour is plenty of time for HIIT—I could barely breathe after about 10 minutes.)

Of course, you (of sweaty ambitions) may be thinking, Why not just keep layering more intense intervals on for a longer, harder workout?

By doing so, you’re essentially putting in extra work for little additional gain—and your time is definitely precious. “What happens is your body just adjusts, so you hold back on the intensity,” Vaziralli says. “You’re spending more time than [necessary] for the same results.” And perhaps most importantly, you’re too busy to deal with an injury—which might happen if you attempt to push to that unsustainable, aggressive intensity for an extended period of time.

Before you schedule seven, 30-minute daily HIIT workouts in a row though, take note: The same “HIIT it and quit it” logic also applies on a weekly basis. “There are countless types and formats of high-intensity training available without tested recommendations on how much is too much,” Jinger Gottschall, PhD told Shape.

Dr. Gottschall—who has rounded up data from HIIT lovers for years—recently partnered with global workout platform Les Mills to find out how much time per week you should actually spend training in your 90 to 100 percent max heart-rate zone in order to yield the best results.

In a small study of 35 people (28 being women), participants were asked to establish their baseline for three weeks, tracking their heart rate and mood throughout their normal workouts. Then, Dr. Gottschall’s team of researchers asked them to complete two 30-minute HIIT workouts, four hours apart.

Based on saliva samples collected before, directly following, and 30 minutes after the cooldown, researchers measured the participants’ cortisol and testosterone levels to see just how beneficial the extra dose of HIIT really was. The results? “I was surprised by the obvious difference between doing 30 to 40 minutes [of HIIT] and doing more than 45 minutes. The difference in performance, stress-related feelings, and sleep quality were significant,” said Gottschall. In other words, too much high-intensity training damages your body similarly to the way overtraining does—yikes!

The bottom line: For the best results, choose between maxing out in shorter workouts or instead opting for longer, sustained cardio and strength-training sessions. And when you do plan out your sweat schedule for the week, make sure super-high intensity sessions only take up a half-hour slot.

Proof that you really can pack a lot of power in a little time: This 10-minute beach workout Halle Berry’s trainer swears by. If you’re reconsidering your fitness routines, try to avoid these 10 gym faux pas the next time you work out. 

Originally published February 28, 2017; updated July 16, 2018, with additional reporting from Kells McPhillips.